Flat-panel displays, such as organic light emitting diode (OLED) displays, of various sizes are proposed for use in many computing and communication applications. In particular, OLED displays are proposed for use in both indoor and outdoor applications under a wide variety of ambient lighting conditions. Indoor applications have relatively low ambient illumination and require lower levels of display luminance. In contrast, outdoor applications can have a high ambient luminance and may require higher levels of display luminance together with low display reflectance. Moreover, most OLED displays are proposed for use in conditions of both high or low to non-existent ambient illumination, from outdoor use during the day to night-time use in a dark room.
Current illumination and display visibility standards cite 75,000 lux as a standard for outdoor illumination on a bright and sunny day. Cloudy bright days have an illumination of 16,000 lux, cloudy dull days have a brightness of 6,000 lux, and a cloudy very dull day has a brightness of 1,000 lux. Indoor illumination ranges from 0 to 1000 lux. Viewability standards for display devices set the minimum display contrast ratio standard for reading text on a display at three. Other sorts of displayed information, such as images, require a higher contrast, for example ten.
Given the wide variety of viewing conditions proposed for OLED displays, it is difficult to design an OLED display having suitable contrast. OLED displays rely on the use of conductive electrodes, typically some form of highly reflective metal, to provide current to an emissive layer of organic material. The reflective metals reflect ambient light to a display viewer, thereby making the display difficult to view. Moreover, an OLED display device includes both light emitting areas and non-light emitting areas. The non-light emitting areas are typically composed of circuitry such as thin-film transistors, capacitors, drivers, and signal lines.
One way of improving contrast in an OLED display device is to use a circular polarizer over the display. The circular polarizer includes a polarizer and a quarter wave plate. The polarizer polarizes ambient light falling on the display, and the quarter wave plate rotates the direction of polarization of the polarized light by 45 degrees. Any polarized light that is subsequently reflected back through the quarter wave plate is further rotated by 45 degrees so that its direction of polarization is orthogonal to the polarizer, and hence is substantially completely absorbed by the polarizer. Circular polarizers absorb approximately 60% of the light that passes through the polarizer once. About 99.5% of the ambient light that is specularly reflected back through the circular polarizer is absorbed. Hence, about 60% of the light emitted by the OLED display device through the circular polarizer is lost, while 99.5% of the ambient light that falls on the surface of the OLED display device is absorbed. Suitable circular polarizers are commercially available, for example from 3M and are described in the patent literature. See for example, WO02/10845 A2 by Trapani et al., published Feb. 7, 2002, which describes a high durability circular polarizer including an unprotected K-type polarizer and a quarter-wavelength retarder that is designed for use with an emissive display module such as an organic light emitting diode or a plasma display device. However, even with the use of circular polarizers, the contrast of OLED devices is not adequate for use outdoors.
A second means of improving contrast in an OLED display device is to place an absorptive layer such as a light absorbing material or a destructive interference layer within a cavity at the back of the device, for example on the substrate or an electrode. See for example U.S. Pat. No. 6,411,019 B1, issued Jun. 25, 2002 to Hofstra et al. The absorptive layer absorbs the ambient light in addition to any light emitted from the emissive layer of organic materials. However, this approach has the difficulty that most of the light emitted from the OLED toward the absorptive layer is lost, thereby severely reducing the brightness of the display.
A third means of improving contrast in an OLED display device is to provide a matrix of light absorbing material called a black matrix between the light emitting elements and around the edges of the display device. See for example, U.S. patent application Ser. No. 2002/0050958 A1 by Matthies, et al., published May 2, 2002. This approach is capable of significantly reducing the reflectance of the display, but still allows substantial ambient light to be reflected from the display by reflection from the reflective anodes of the light emitting elements.
There is a need therefore for an improved OLED display device that has improved contrast.